ephy-shell: Don't open new tab on startup if there are downloads in progress

Fixes GNOME/epiphany#1058
3 jobs for DontOpenNewTabOnStartupIfActiveDownloads in 10 minutes and 9 seconds (queued for 1 second)
Status Job ID Name Coverage
  .Pre
passed #592023
check-code-style

00:01:17

passed #592024
flatpak-scanbuild

00:05:34

 
  Build
passed #592025
flatpak

00:04:33